1. Field of the Invention
The present invention relates to the finishing of surfaces of articles intended for relative motion with respect to a fluid, i.e. gas or liquid, environment such as air or water, and particularly to the surfaces of airfoils. It is especially useful in finishing the wings of aircraft.
In finishing the surfaces of an aircraft wing, three of the major considerations are: smoothness of the finish, general correspondence of the surface to an optimum configuration, and resistance to abrasion.
The first of these is of extreme importance in connection with the wings of metal (usually aluminum) aircraft whose surfaces contain many irregularities caused by rivets, seams, etc. Even one such surface irregularity can drastically reduce the efficiency of the wing if it is located between the leading edge and the boundary layer separation point of the wing. As is well known in the art, aircraft wings, particularly those designed for low speed aircraft, are configured so as to cause the air through which they move to flow along the airfoil surface from the leading edge to a point known as the boundary layer separation point. From this point to the trailing edge of the airfoil, the air no longer follows the contour of the airfoil but becomes turbulent. Surface irregularities between the leading edge and the boundary layer separation point, especially those which are disposed transverse to the direction of air flow, interrupt the flow of air along the airfoil surface creating turbulence and increasing drag on the wing. One ramification is that the aircraft must then use an excessive amount of fuel. The turbulence created by surface irregularities can also generate flutter phenomena, and vibration and resonance problems. Furthermore, it will be appreciated that an irregular, as opposed to a smooth, surface experiences more frictional heat as it moves through a fluid. It is thus of extreme importance that the finish of an aircraft wing, especially at an near the leading edge, be as smooth as possible.
The second consideration, correspondence to an optimum configuration, is somewhat similar in that it too relates to the efficiency of the wing. In designing a wing for a particular aircraft, an optimum configuration, represented by a mathematical model or curve, is determined. This optimum configuration is designed to provide maximum efficiency and, specifically, to maximize the lift-over-drag coefficient (L/D). In constructing the wing, it is important to see that the outer surface conforms to the optimum configuration as nearly as possible. Again, this is most important at and near the leading edge of the wing where even the slightest deviation from the optimum configuration can reduce wing efficiency. Unfortunately, it is virtually impossible to construct a wing which corresponds perfectly with the mathematical curve, especially when working with metal. A metal wing is usually provided with a number of internal support beams and an outer skin of sheet metal. The metal skin tends to dip in the areas between beams and form shallow concave deviations from the optimum configuration.
The third consideration, resistance to abrasion, is also most critical at and near the leading edge of the aircraft wing which tends to become worn by dust and other suspended particles in the air through which it moves. Abrasion of the surface results in irregularities, the problems of which are discussed above.
As mentioned above, surface finish and correspondence to the optimum configuration are of particular importance in the wings of low speed aircraft in which the air must flow along a substantial portion of the surface to provide lift for the aircraft. In high speed aircraft, i.e. those which typically cruise at speeds in excess of about 250 to 275 m./sec. (meters per second), the airfoil shape of the wing is not critical during high speed operation so that these first two problems are not as severe. However, the problems do still exist to a certain extent and, of course, the problem of abrasion at the leading edge is present. Indeed, it will be appreciated that smooth, abrasion resistant surfaces which correspond as nearly as possible to their optimum configurations are advantageous, to varying degrees and for various reasons, in virtually any article which moves relative to a fluid medium. For example, the hulls of boats would benefit from such surfaces.
2. Description of the Prior Art
In the past various methods of combating the above-mentioned problems have been devised. To provide a smooth surface conforming closely to an optimum configuration, the wings of low speed aircraft such as racing gliders have sometimes been constructed of materials such as fiberglass which are molded and hand finished by sanding, polishing, etc. This provides a relatively smooth and true surface. However, the hand finishing is tedious, time consuming and expensive and requires special skill. Additionally, there is a great weight penalty in using relatively heavy wings of solid fiberglass.
Aluminum wings are often used because of their relatively light weight. However, as mentioned above, they necessarily contain surface irregularities such as seams and rivets as well as concave deviations from the optimum configuration. These irregularities and concavities have sometimes been eliminated by skilled workers spreading a filler material over the irregularities and/or concavities, allowing it to harden, and then sanding it, preferably with an instrument having a straight edge. As in the hand finishing of fiberglass wings, this is time consuming and expensive.
U.S. Pat. No. 3,607,595 discloses another method for alleviating surfaces irregularities in metal wings by covering them with a hardenable foam sealed with a thermosetting resin. It is believed that relatively highly skilled workers and/or expensive and sophisticated equipment would be needed to practice this method. In any event, it is apparent that the method would affect only relatively small surface irregularities and would be of little use in correcting larger deviations from the optimum configuration.
U.S. Pat. No. 2,973,170 teaches the coating of an aircraft wing with porcelain. Again, it is noted that this would appear to require a relatively high degree of sophistication in the workers and/or the equipment used and might only serve to correct minor surface irregularities. Additionally, it is noted that porcelain is a relatively heavy material and thus undesirable for use in aircraft.
None of the above methods, except possibly the last mentioned, are useful in providing resistance to abrasion, and other techniques have been used for this purpose. Generally, these have consisted of hardsurfacing the leading edges of the wings. A common hardsurfacing material is an epoxy heavily impregnated with tungsten carbide powder. The material is quite hard and resistant to abrasion. However, it is also very brittle so that when it does wear, it tends to break away in chunks rather than wearing gradually. This leaves distinct cavities in the surface which render the wing highly inefficient and even dangerous. Furthermore, the hardsurfacing techniques, like the finishing methods described above, require sophisticated workers and/or equipment and are thus expensive.